The invention relates to a method for preserving fruits and vegetables which are prepared and packaged ready for consumption, especially geophytes and mushrooms, by means of gases introduced into the packing container or bag.
Foodstuff has been preserved by conserving gases for some time but the practice has gained rapid importance as the consumers are asking for more fully prepared foodstuff packaged for ready consumption. Such foodstuff includes all kinds of meats, that is, meat products, fish, cheese, pastas and vegetables and also various mixtures of such foodstuff. For the preservation of such foodstuff for a period of at least several days, a mixture of gases has been utilized, preferably carbon dioxide and nitrogen. In order to avoid changes in the appearance of the foodstuff, that is, in order to retain its fresh appearance, in some cases, particularly in connection with fresh meat, oxygen is added to the gas introduced into the packaging container which causes reddening of the myoglobin. The addition of oxygen however shortens the preservation period of the meat even if the package is cooled. For the preservation of foodstuff it is necessary to destroy the microorganisms on or within the foodstuff or at least to prevent their growth. In addition enzymatic or physical and chemical changes in the foodstuff should be prevented. During the conservation period the foodstuff should retain as much as possible its consistency and appearance and also its nutrition values and its flavor.
As far as the destruction, or prevention of growth, of the microorganisms is concerned, special attention must be directed to the aerobic bacteria, that is, bacteria which need oxygen to grow and exist. Those bacterias are therefore generally inhibited by keeping out the oxygen they need to exist, for example, by replacing the oxygen in the surrounding air by gas mixtures which do not provide for the gas exchange needed by those bacterias. This is possible, for example, with pure nitrogen but it is necessary that the package bags or containers contain really only nitrogen. However also carbon/oxygen compounds, especially carbon dioxide (CO.sub.2), are suitable gases and also mixtures of carbon dioxide and nitrogen. Also carbon monoxide would be a suitable gas for the prevention of bacteria growth; it can be used however only in small amounts since it is not without danger for humans. Depending on the goods to be preserved and on the period for which the goods have to be preserved, more or less oxygen may be added to, or may be left with, these gases or gas mixtures. With cooled fresh meat the oxygen content may be up to 70% with about 20% carbon dioxide and 10% nitrogen. Other mixture ratios may be utilized of course but the given limit of 70% oxygen is not exceeded, not even with goods that have to be preserved for only relatively short periods of time; bacteriological protection is obtained herein only with the protective gases nitrogen (N.sub.2), carbon dioxide (CO.sub.2) and carbon monoxide (CO).
Before the goods to be preserved are surrounded by such gases they may be cooled or frozen. Freezing to a temperature of substantially below 0.degree. C. however is not suitable for all types of foods. Vegetables and fruits, for example, may generally not be cooled to below 0.degree. C. An exception are vegetables which are cooked before consumption.
For the preparation of salads, only fresh foods may be used, that is, the various kinds of vegetables: escarole, radicchio, cabbage, romaine, herbs, sweet corn, radishes, celery roots, carrots, potatoes, onions and so on. Recently however also vegetables are prepackaged as salads and salad mixtures ready for consumption. Those salads too are packaged under a protective cover gas and since they are quite easily spoiled they are packaged with nitrogen or nitrogen-carbon dioxide gas mixtures. This is also true for mushrooms which recently also appear on the market more frequently. But in spite of refrigeration, which is used in addition to the packaging under a protective gas, these sensitive vegetables will generally nevertheless suffer losses in quality as far as color and taste are concerned already after a relatively short storage time of only a few days.
It is the object of the present invention to provide a method which avoids the relatively rapid losses in quality of such sensitive foods.